This invention generally relates to a method and apparatus for controlling the operation of a programmable logic array and, more specifically, relates to a control circuit particularly adapted for directing the operation of a communication terminal.
Communication terminals are widely used as devices for the reception and transmission of encoded data. Such terminals frequently include at least one information display in the form of a printer which provides the operator with a "hard copy" of the data received or sent. A type of "hard copy" printer finding increased application is the high speed matrix printer such as that described in U.S. Pat. No. 3,973,661 to DeBoo et al. entitled "Wire Matrix Printers and Electromagnetic Actuator Mechanisms Useful in Such Printers" issued Aug. 10, 1976. Such printers utilize a print head having a vertical array of print wires. Selected print wires, corresponding to a particular portion of a selected character, are actuated at each of the several head locations during the printing of a single character. The individual characters are printed as the print head is stepped across the copy during the printing of a text line. The selection of the print wires as well as the operation of the motors which position the print head across the paper and control the advance of the paper require relatively complex, sequentially timed control circuitry. One approach to such a control arrangement includes a microprocessor, several integrated circuits providing input-output and support functions, and a storage memory. The limited instructional set available with such microprocessors and their sequential command format generally necessitates their use with large and costly storage memories. Consequently, programming costs are high and should a change in the operation of the printer be required to accommodate a unique application, a substantial reprogramming effort is required. Further, the inherent time delays incurred while waiting for the motors to change direction, reach operating speeds and complete the desired operations usually require the use of separate timing devices in combination with the microprocessor.
The illustrated embodiment utilizes a programmable logic array (PLA) which features an unlimited instructional set thus providing high programming flexibility. Such an instructional set includes parallel in addition to sequential commands. The parallel command capability increases the effective speed of the control system and reduces the amount of memory capacity necessary to implement a selected program. Further, the number of support components, as compared to the microprocessor implementation of the same program, is considerably reduced. In practice, the entire control circuit of the illustrated embodiment may be incorporated into a portion of a single integrated circuit package. Additionally, program changes and modifications are easily incorporated without extensive and time consuming program revisions.
As previously mentioned, printer control usually requires the adherence to specified time delays on a real time basis to allow certain of the motors and other electromechanical devices in the printer to accomplish specified functions before subsequent operations may be performed. With reference to the illustrated embodiment, such timing delays are incorporated into the PLA control by the use of a clocked counter addressing one input field of the PLA thereby eliminating the necessity of providing separate peripheral timing components. As will subsequently be more fully appreciated, the real time operation of the illustrated embodiment uniquely accommodates the operational characterstic of a communications printer.
Thus, a control apparatus is described which is relatively easy to program and effectively controls the operation of a printer or other device with a minimum of operational instructions and corresponding low execution time.